In recent years, as a cloud service comes into wide use, data amount of companies or individuals which are accumulated in a data center is rapidly increased. Since the data collected in a cloud is not under the management of a user, some users may concern about that the data is illegally used. For this reason, from the viewpoint of data security, there is a requested method which can obviously show the user data operation/management state such as erase of data accumulated in a storage device used in the cloud service.